This invention relates to a new high strength lead alloy. While an alloy according to the invention finds particular utility as a grid and/or top lead material for use in lead-acid storage batteries, certain properties of the alloy make it attractive for use in other applications as well where a high strength lead is desired.
Lead-acid storage batteries typically employ lead alloys containing antimony as a primary constituant due to the effect of antimony on the physical properties of the lead. Antimony is used to increase the strength and/or other physical properties of lead, facilitating various aspects of battery manufacture. In the case of lead-acid battery grids, this is particularly important in order for the grids to withstand normal handling during battery manufacturing and service.
Recently the battery industry began producing batteries which require little or no maintenance such as adding of water to maintain the electrolyte level during the service life of a battery. In such batteries it is the practice to either seal the battery or use vent plugs for the filling ports which are not easily removed by the ultimate battery user. Since the purpose of such batteries is to eliminate the need for filling, a lead alloy system must be selected in which the supply of electrolyte will not be significantly diminished over the intended life of the battery. The presence of antimony typically causes excessive gas generation in lead-acid batteries, especially during periods of charging or overcharging, which ultimately depletes the quantity of electrolyte. In addition, excessive gassing is unacceptable in reduced or no-maintenance batteries if they are of the completely sealed type. Conventional alloys for this type of battery contain calcium in place of antimony. Calcium alloys reduce gas generation. Examples of lead-calcium alloys are seen in the following U.S. Pat. Nos. 3,920,473 issued Nov. 18, 1975 to Sims; 3,881,953 issued May 6, 1975 to Turowski; 3,287,165 issued Nov. 22, 1966 to Jensen; 2,794,707 issued June 4, 1957 to Walsh; 2,159,124 issued May 20, 1937 to Betterton et al; and 1,703,212 issued Feb. 26, 1929 to Shoemaker.
A disadvantage of the lead-calcium system is that its alloys do not generally have mechanical properties comparable to lead-antimony alloys. Since the battery industry is continually striving to make battery grids with smaller cross-sections than previously used, the strength of the lead-calcium grid alloys becomes a limiting factor in grid design. Lead-calcium systems also have a propensity to grow mechanically or creep as is well known in the art. If creep becomes excessive within a battery, adjacent parts may short out thereby disabling the battery or seriously reducing its capacity. Creep may become excessive with minor alloy compositional changes.
A further disadvantage of conventional alloys lies in the limitations on the re-use of lead scrap due to decreased pot stability. For example, melting of the scrap results in excessive drossing with a resultant loss of the calcium in the alloy. Adjustments must therefore be made in the re-melted scrap to bring the alloy composition back into a desired range prior to casting. Furthermore drossing of lead-calcium increases during mechanical agitation of the molten alloy both initially and for re-melted scrap thereby necessitating protective measures such as the use of shielded pots.
Strontium has been proposed for use in lead alloys by others. For example, U.S. Pat. No. 1,158,672 issued Nov. 2, 1915 to Frary et al, discloses a lead alloy containing a plurality of alkaline earths including calcium, barium, strontium and magnesium for use in bullets. U.S. Pat. No. 2,013,487 issued June 7, 1934 to Canfield et al, discloses a lead-strontium-tin alloy for use in lead-acid battery grids, and U.S. Pat. No. 2,040,078 issued May 12, 1936 to Canfield et al, shows a lead-strontium alloy. Finally U.S. Pat. No. 2,170,650 issued Aug. 22, 1939 to Bouton et al, shows a lead-calcium-barium-strontium alloy. These patents do not show lead alloys which have all the advantages displayed by applicant's invention however, such as dross protection, pot stability on re-melt and high strength.